1. Field of the Invention
The present invention relates to an ultraviolet-curing covering composition which forms a coating film having excellent hardwearing properties after curing. Particularly, the present invention relates to an ultraviolet-curing covering composition which forms a coating film having excellent adhesive properties, transparency and hardwearing properties. More particularly, the present invention relates to an ultraviolet-curing covering composition which forms a coating film upon evaporation of solvents, and which can be subjected to processing such as molding, printing and transfer before curing by ultraviolet light and can give a hard-coating having excellent hardwearing properties after curing.
1. Description of the Background Art
Plastic articles, for example, polycarbonate, polymethyl methacrylate, polyethylene terephthalate, polyvinyl chloride, ABS resin and the like have a variety of applications because their properties of lightness, easy processability, impact strength and the like are excellent. However, these plastic articles get scratched readily because of their low surface hardness. For example, transparent resins such as polycarbonates have disadvantages in that their inherent transparency or appearance is markedly lost; thus it is difficult to use these plastic articles in fields where hardwearing properties are required. For this reason, ultraviolet-curing hard coating materials are being sought which can impart hardwearing properties to these plastic articles and can increase their surface hardness.
When an ultraviolet-curing composition is applied to surfaces of these plastic articles, it is desirable that before curing, a coating layer can be processed or a coating layer which is coated on another substrate and, if necessary, printed, coated with an adhesive can be transferred to the surface of a plastic article. Therefore, an ultraviolet-curing covering composition which forms a coating film upon evaporation of solvents and imparts excellent hardwearing properties after curing has been sought.
Various hard coating compounds such as acrylic hard-coating compounds and silicon hard-coating compounds have been proposed as covering materials which can impart hardwearing properties to these plastic substrates and can increase surface hardness. Among these hard coating compounds, acrylic hard-coating compounds are characterized by shortened curing time and excellent productivity as they can be cured by irradiation of ultraviolet light, but they have the disadvantage of poor hardwearing properties compared with silicone hard-coating compounds.
To improve the hardwearing properties of these acrylic hard-coating compounds, Japanese Patent Publication (PCT) No.57-500984 disclosed an ultraviolet-curing covering composition in which silica sol in water as a dispersing medium and hydrolyzed silane compounds having acryloxy-functionality or glycidoxy-functionality are added to acrylate. However, as the silica sol used in the above-mentioned invention is water-dispersive there is a limitation in that a hydroxyl group-containing acrylate which has good miscibility with water must be used as an acrylate so that sufficient hardwearing properties can not be obtained.
Japanese Patent Publication Nos. 53-43553 and 57-20968 disclose hard coating compounds using polyfunctional acrylate having three functional groups or more to overcome the above-mentioned problems. Further, Japanese Patent Laid-Open Nos. 59-41366 and 63-92675 disclose that inorganic fillers and/or colloidal silica are added to these hard coating compounds to improve their hardwearing properties. Japanese Patent Laid-Open No. 59-204669 discloses a coating composition with hardwearing properties, in which a hydrophobic silica sol in an organic solvent as a dispersing medium and a hydrolyzed silane compound having unsaturated groups are added to a polyfunctional acrylate. As a hydrophobic silica sol in an organic solvent as a dispersing medium is used in this coating film composition, the selection range of applicable polyfunctional acrylates can be enlarged to further improve the surface hardness of plastic articles. However, the unsaturated group-containing silane compounds used in this composition such as 3-(meth)acryloxypropyl trimethoxysilane can not yet impart sufficient hardwearing properties equal to those of silicone hard coating compounds.
Further, as these ultraviolet-curing hard coating compounds, irrespective of containing solvents, do not have film-forming properties upon evaporation of the solvent, processing such as molding, printing and transfer of coating films could not be performed before irradiation of ultraviolet light.
Even though blending thermoplastic resins such as copolymers of (meth) acrylic ester may be considered to impart film forming properties to these hard coating compounds, the addition of just amounts to impart sufficient film-forming properties would greatly decrease the hardwearing properties, so hard coating compounds which are excellent in both film-forming properties and hardwearing properties have not yet been achieved.